Mitochondrial distribution and function in oocytes and early embryos

Abstract

The frequency of chromosome abnormalities due to non-disjunction of maternal chromosomes during meiosis is a function of age, with a sharp increase in the slope of the trisomy-age curve between the ages of 30 and 40 years. The basis of this increase, which is a major cause of birth defects, is unknown at present. In recent years, mutations in mitochondrial (mt) DNA have been associated with a growing number of disorders, including those associated with spontaneous deletions of mtDNA (ΔmtDNAs). Intriguingly, these pathogenic ΔmtDNAs, which are present at extremely high levels in certain patients, are also present at extremely low levels (detectable only by polymerase chain reaction) in normal individuals. The proportion of such ΔmtDNAs in normal muscle is a function of age; the shape of this curve is exponential, with the accelerating part of the curve beginning at 30-40 years. We postulate that, as well as muscle and brain, a similar time-dependent accumulation of ΔmtDNAs also occurs in normal oocytes. Since ΔmtDNAs are functionally inactive, an accumulation of such aberrant genomes could eventually compromise ATP-dependent energy-utilization in these cells. Furthermore, these deficiencies would also affect the function of the somatic follicular cells that surround, and secrete important paracrine factors to, the oocyte. If there is indeed an age-associated relationship between ΔmtDNAs and oocyte age, perhaps errors in meiosis (which is almost certainly an energy, and ATP, dependent process) are related to mutations in mtDNA (primarily deletions, but perhaps point mutations as well) in oocytes and/or the surrounding somatic cells, which result in deficiencies in both mitochondrial function in general and oxidative energy metabolism in particular. This hypothesis wo uld explain many of the non-Mendelian features associated with maternal age-related trisomies, e.g. Down's syndrome. © Oxford University Press 2001.